In recent years, a strong demand for downsizing electronic devices requires electronic components to be downsized. For example, filters and duplexers using acoustic waves are required to be downsized. Exemplary acoustic wave elements using acoustic waves are surface acoustic wave (SAW) elements using surface acoustic waves, film bulk acoustic resonator (FBAR) elements using piezoelectric thin films, and the like.
To meet the demand for downsizing electronic components, wafer level packages, which use a resin instead of a package to seal an element located on a substrate with the resin provided so as to cover the element, have been developed. In addition, when an acoustic wave element is used as an element for example, a cavity located on a functional part of the acoustic wave element is necessary to maintain characteristics of the acoustic wave element, and this may be met by a structure designed to have a resin portion having a cavity on the functional part of the acoustic wave element, which is a so-called hollow structure, for example. The functional part of the acoustic wave element is electrode fingers of a comb-shaped electrode (IDT: Interdigital Transducer) in a surface acoustic wave element, and is a region where upper and lower electrodes, which sandwich a piezoelectric thin film, overlap each other in a piezoelectric thin film resonator element.
The resin portion having the cavity on the functional part of the acoustic wave element may be formed by a method that bonds a resin sheet including a concave portion formed therein, which is to become the cavity, to a substrate including the acoustic wave element formed thereon as disclosed in Japanese Patent Application Publication No. 2005-175345 (Patent Document 1). In addition, there has been known a method that forms a resin portion having the cavity on the functional part of the acoustic wave element, and forms a metal layer in the resin portion located on the upper side of the cavity as disclosed in Japanese Patent Application Publication No. 2008-227748 (Patent Document 2).
Patent Document 1 may fail to obtain the cavity having a sufficient strength because the height is reduced as the size is reduced. On the other hand, the strength of the cavity can be enhanced by providing the metal layer in the resin portion located on the upper side of the cavity as disclosed in Patent Document 2.
However, Patent Document 2 forms the cavity by bonding the resin including a metal layer to a piezoelectric substrate with an acoustic wave element formed thereon, and this may cause misalignment of the metal layer with respect to the acoustic wave element in height and horizontal directions because the resin is soft. When the reduction in height shortens the distance between the acoustic wave element and the metal layer, the misalignment of the metal layer causes a change in capacitance, and may degrade characteristics. In addition, as the resin supporting the metal layer is located on a signal wiring, a capacitor between the signal wiring and the resin may degrade characteristics.